We report on a case of paraneoplastic pemphigus associated with Castleman's disease. Clinical pathologic features were not conclusive. Diagnosis was established thanks to the detection of seric autoantibodies directed against intercellular substance by indirect immunofluorescence on monkey esophagus. The positive result of this test prompted us to reevaluate the patient and to detect the occult neoplasia. The demonstration of autoantibodies against plakins is the key marker of this disease but depends on tests that may not be readily available in many places like immunoprecipitation, immunoblotting, or indirect immunofluorescence over rat bladder. In this setting, tests like indirect immunofluorescence over monkey esophagus, although unspecific, may aid in reaching the appropriate diagnosis. This case illustrates the importance of the laboratory of autoimmunity in the diagnosis of this type of pemphigus.